Page 185 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES
SULPHATES
*BIREFRINGENCE High (much greater than gypsum) with third order colours. n. = 1.636
*INTERFERENCE Seen on an elongate prism section, a Bx. figure is just bigger than field of
FIGURE n p = 1.637
view and positive.
n, = 1.647
*EXTINCTION Straight on all cleavages and prism edges. 8 = 0.012
TWINNING Repeated on {011}.
2V, = 37° +ve (length slow)
*OCCURRENCE Similar to gypsum, being found in evaporite deposits. Anhydrite may OAP is parallel to (010). Crystal is elongated parallel to the a axis
form by hydrothermal alteration of limestones or dolomites. D = 4.5 H = 2%-3% (barite)
D = 4.0 H = 2%-3V2 (celestite)
Barite (baryte or barytes) BaSO.
orthorhombic
Celestite (celestine) SrS0 4 coLouR Colourless.
0.815 :1:1.314
HABIT Subhedral clusters of prismatic crystals common.
Celestite (celestine) c=o. cLEAVAGE Basal cleavage { 001} perfect, { 210} and { 010} cleavages present.
RELIEF Moderate.
BIREFRINGENCE Low, first order yellows, but mottled colours are common.
*INTERFERENCE Bx. figure with small 2V seen on (100) section, i.e. section with two
FIGURE cleavages.
EXTINCTION Straight on cleavage trace or prism edge.
TWINNING Lamellar twinning present on { 110}.
*OCCURRENCE Barite occurs as a gangue mineral in ore-bearing hydrothermal veins, in
association with fluorite, calcite and quartz. Barite occurs as stratiform
deposits of synsedimentary exhalative origin in sedimentary and
metamorphic terrains.
Celestite SrS0 4 is similar to barite optically and occurs in dolomites, in
evaporite deposits, and rarely in hydrothermal veins. There is probably
a complete solid solution series from barite to celestite.

c=o.
Barite

/
/
/
/
/
a ='Y
172
173